Your scenario is too simplistic, someone would notice such effects. :-)

What you need to do first is convince everyone that their instruments always must be corrected to the sea level pressure.
Such correction of course involves mathematics that most laymen do not understand and those who do, do not see the hidden beauty of the trickery, after all it will give an accurate elevation reading which is what most people that pilot airplanes need.

Note that when using a weather barometer (usually with a scale range between 28 and 31 inches Hg) at a location above sea level, the reading must be corrected back to sea level. This is automatically accomplished when you initially match your barometer's reading to that reported by local TV or radio weather forecast. These reported readings have already been "corrected" to sea level, thus eliminating any pressure differences due to elevation.

http://www.sciencecompany.com/barometric/barometer.htm

Second of all, you need to eliminate all mercury barometers so that no one can compare the present pressures to the old measurements.

Third you need to somehow enforce peoples perception that the Earth's atmosphere has, is, and will be stable in a global scale for billions of years to come.

Then you rely on the laziness of the people, they will not question scientists.

The only way that I see that this dilemma can be satisfied is to see a mercury barometer at sea level.
Simple solution, if the mercury barometer at sea level reads the same today as it did 150 years ago, then everything is fine and there is no decrease in atmospheric density.